1. Field of the Invention
The invention relates to new isocyanurate group-containing polyisocyanates based on selected diisocyanates, a process for the production thereof by the trimerization of a proportion of the isocyanate groups of specific aliphatic-cycloaliphatic diisocyanates and the use of the isocyanato isocyanurates as the isocyanate component in polyurethane lacquers.
2. Description of the Prior Art
Isocyanurate group-containing (cyclo-) aliphatic polyisocyanates are of great interest industrially as lacquer polyisocyanates. Whereas low molecular weight aliphatic diisocyanates cannot be used as such in lacquer binders owing to their toxicological properties and their relatively high volatility, isocyanato isocyanurates have a number of advantages. They are oligomers with a low vapor pressure which are substantially free from monomers. They have an isocyanate functionality of .gtoreq.3 so that a high degree of cross-linking can be achieved. As aliphatic polyisocyanates, they provide good light-fastness in the coatings obtained from them.
Various processes are known for producing isocyanurate group-containing polyisocyanates. These known processes generally differ in the selection of the trimerization catalysts or in the selection of polyisocyanates to be used during the trimerization reaction (see, for example, GB-P 1,391,066, GB-P 1,386,399, DE-OS 2,325,826, DE-OS 2,616,415, DE-OS 2,806,731, DE-OS 2,901,479, DE-OS 3,100,262, DE-OS 3,219,608, EP-OS 17,998, EP-OS 33,581, EP-OS 57,653, EP-OS 89,297, EP-OS 82,987 or EP-OS 100,129).
All of these processes have a substantial disadvantage. In order to obtain low viscosity isocyanato isocyanurates or isocyanurates which are readily soluble in lacquer solvents, it is necessary to interrupt the trimerization reaction at a relatively low degree of trimerization (degree of trimerization=percentage of the isocyanate groups present in the starting isocyanates which have been trimerized) and to carry out thin film distillation, which generally involves complex apparatus, for obtaining a trimerization product which is low in monomers. Otherwise, relatively large quantities of oligoisocyanurates (polyisocyanates with isocyanurate structure containing more than one isocyanurate ring per molecule) would be formed in addition to the low viscosity and readily soluble monomeric isocyanurates which are predominantly desired. This is due to the fact that the isocyanate groups of the starting diisocyanates generally have similar or only slightly different reactivity so the possibility of both isocyanate groups of the starting diisocyanates reacting to form isocyanurates cannot be ruled out.
It was accordingly the underlying aim of the invention to provide new isocyanurate group-containing polyisocyanates which are suitable as lacquer polyisocyanates and which contain substantially no oligomeric trimerization products of the above-mentioned type, i.e., which are composed substantially of tris-isocyanato-monoisocyanurates, and during whose production the trimerization reaction does not have to be interrupted prematurely, i.e., at a degree of trimerization of from about 20 to 40%, in order to achieve this objective.
This aim may be achieved with the isocyanato isocyanurates according to the invention as described in more detail below and by the process for the production thereof in which quite specific starting diisocyanates are used.